With the recent diversification of articles to be packed, a variety of packaging films have been developed. As packaging films, there are known, for example, highly strong films, highly elastic films, resilient films, adhesive films, transparent films, electrically conductive films, light-shielding films, gas-barrier films, heat resistant films, chemical resistant films, composite films obtainable by combining the films above-mentioned, and the like. These packaging films are considerably different from one another in the characteristics of constituent polymers thereof. It is therefore difficult that a one-type film simultaneously satisfies a plurality of characteristics. Accordingly, a variety of packaging films are respectively used according to the purposes.
A liquid crystal polymer classified in engineering plastics is excellent in mechanical properties, dimensional stability, heat resistance, chemical stability and electric properties. Thus, attention is given to the liquid crystal polymer as a polymer film satisfying a variety of requirements. Japanese Patent Publication No. 42287/1985 describes that the liquid crystal polymer film may be divided to form highly elastic split fibers. As apparent from this description, such a film tends to orient in the polymer flowing direction, and is extremely weak in strength in the widthwise direction (hereinafter referred to as TD direction) at a right angle to the film drawing direction (hereinafter referred to as MD direction). To improve the strength of the liquid crystal polymer film in the TD direction, Japanese Patent Application Laid-Open No. 46728/1981 and No. 102234/1986 disclose a method of preparing a biaxially oriented film by increasing the blow ratio at the time when the film is molded by an inflation method. Japanese Patent Application Laid-Open No. 2127/1981 and No. 173620/1988 disclose a method in which a ring die is rotated at the time when a liquid crystal polymer film is molded by an inflation method. Japanese Patent Application Laid-Open No. 59818/1983 discloses a method in which a porous plate-like member having a plurality of fine gaps which can be heated, is formed inside a T-die. Further, Japanese Patent Application Laid-Open No. 31729/1988 discloses a method using a three-layer co-extrusion die, by which both external layers outside of the intermediate layer of a liquid crystal polymer are formed by thermoplastic polymers which are not adhesive to the liquid crystal polymer, and, after the three layers have been co-extruded, the external layers are separated, thereby to obtain the liquid crystal polymer film of the intermediate layer.
However, the thus obtained liquid crystal polymer films are insufficient in thermal adhesive properties (heat weldability). It is therefore not possible to pack an article with such a film by heat seal. Thus, the application of such a film is considerably limited.
It is an object of the present invention to provide a method of manufacturing a laminated film having improved strength in the TD direction thereof.
It is another object of the present invention to provide a method of a laminated film excellent in thermal adhesive properties while the excellent characteristics of a liquid crystal polymer are utilized.
It is a further object of the present invention to provide a method of economically and industrially manufacturing a laminated film suitable for a packaging film or the like.